Police have called for a postmortem on Nguyen Thi Thu Phong, 34, who died on Saturday night after receiving treatment for inflammation of the uterus at the Maria private health clinic, district police chief Bui Van Dai told dpa.

"We will treat it as a criminal case if necessary," he said.

Clinic staff prevented Phong's relatives from seeing her on Saturday, later telling them she had died of unknown causes during treatment.

Three men and one woman employed at the centre, all Chinese nationals, fled the clinic following the incident, local media quoted police as saying.

Three of them had reportedly examined and treated Phong, while a fourth was in charge of gynaecology at the clinic.

Police chief Dai said his department has asked immigration officials to stop the four from leaving the country.

Just two of the medics were licensed, and only as assistants, said Nguyen Khac Hien, director of the Department of Health in Hanoi.

The clinic had already been fined for making exaggerated claims in its advertisements and overcharging patients.

Foreign medical professionals need a license from Vietnamese health authorities before they can practise medicine in the country.